🛡️ Sentinel: Fix sub-process spawn error handling to prevent hangs

[piyyy314]

This PR improves the robustness of subprocess execution within the TypeScript CLI codebase.

Previously, src/lib/sub-process.ts had no event listener for the 'error' event on the spawned ChildProcess. This meant that if spawning the child process failed (e.g. command not found or execution permissions issues), Node.js would throw an unhandled exception or leave the returned promise permanently pending, causing a potential hang or crash.

We resolved this by:

1. Adding a 'error' event listener on the spawned process (proc.on('error', ...)) to reject the promise gracefully.
2. Adding dedicated Jest unit tests under test/jest/unit/lib/sub-process.spec.ts to cover successful execution, non-zero exit codes, and process spawning failures.

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PR created automatically by Jules for task 2534864274848222602 started by @piyyy314

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[piyyy314]

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